I've worked with the HP34401's and never seen that. Would drive me nuts
You can program the comma separator on/off.
In the eBay ad, looks like segment E shorted to colon? The colon is on with any digit that has segment E on.
I would say trouble with the VFD driver IC or solder bridge on the PCB. NEC uPD7527 to drive the VFD.
Hope I'm wrong?